The success of the advanced ceramics industry revolves around the ability to reproducibly make high quality pieces. These pieces are the result of a long series of production steps, e.g., powder synthesis, precipitation, drying/calcining, dispersion, green body formation, binder burnout, and sintering. The quality of the product from each of the aforementioned steps is limited by the quality of the product of the previous step. As such, the synthesis and characteristics of the ceramic powder sets the upper limit for the quality of the final article produced.
One of the most desirable powder properties is the ability to pack efficiently. This ability controls both the green and the fired density of the product, influences microstructural development, and controls the degree of shrinkage upon sintering. For many advanced ceramic applications, the ideal powder would be composed of monodispersed, spherical, submicron particles exhibiting minimal agglomeration.
Powders of most metal oxides are commercially available from a variety of sources. These metal oxide powders, however, suffer from comparatively larger particle sizes (e.g., 0.5 to several microns), polydispersity, and the presence of particle agglomerates. The powders are additionally restricted by their inability to mix on the "molecular level", in the formation of the more complex composite ceramics.
The present inventor believes that colloidal sols of zirconium oxide can be substituted for powdered zirconium oxides in the production of advanced ceramic pieces. The advantage of using a colloidal preparation is the production of monodispersed particles in the submicron range (i.e., down to several nanometers). As a colloidal product the agglomeration associated with a dried/calcined product is avoided by continuous dispersion in solution. This gives sols increased ease of use for surface coatings and gives them the capability for the "molecular mixing" necessary for custom composite formulation.
Various attempts have been made to grow zirconia particles from solution, such as that set forth in Japanese Patent No. 87/162,624; Blier, A. and Cannon, R.M., "Nucleation and Growth of Uniform m-Zro.sub.2 ", Better Ceramics Through Chemistry II: Proceedings of the Materials Research Society Symposia Proceedings, Apr. 15-19, 1986; and U.S. Pat. No. 4,612,138 (B. Keiser), issued Sept. 16, 1986.
Japanese Patent No. 87/162,624 discloses the production of fibrous bundle-shaped coagulated particles of monoclinic zirconia crystals by agglomerating monoclinic zirconia crystals along the C axis to give microthin fiber shape. These crystals are prepared by hydrothermal reaction of soluble zirconium salt or compounds containing 0.1-2.0 mol/L of zirconium and 0.2-2.0 mol/L of SO.sub.4.sup.2- with an acidic aqueous solution containing 0.05-1.5 mol/L of Mg.sup.2+ or NH.sub.4 + at 120-300.degree. C. Thus, 16 g ZrOCl.sub.2.8H.sub.2 O was dissolved in water, then mixed with 10 mL concentrated NH.sub.4 OH to form a hydroxide precipitate, filtered, and washed. The resulting precipitate, still containing NH.sub.4 OH traces, was dissolved in 36 N H.sub.2 SO.sub.4 to give a solution containing 0.25 mol/L of Zr, 0.5 mol/L of SO.sub.4.sup.2-, and 0.1 mol/L of NH.sub.4 +. The solution was heated in an autoclave at 150.degree. for 2 days to react and form white fibrous bundle-shape zirconia particles (width 1000A and length 8000 A). The reactions of each example were run in TEFLON lined reactors.
The Blier et al. article discloses the synthesis of aggregate particles from Zr(O)(NO.sub.3).sub.2 with most of the synthesis under non-hydrothermal conditions. The process runs at a much lower concentration and for a longer period of time, i.e., 24 to 72 hours, than the present invention.
The Keiser patent discloses a process for forming a stable acidic and alkaline metal oxide sol by hydrolyzing a metal salt precursor in the presence of a stabilizing component chosen from the group consisting of surfactants having an HLB of at least 8, dispersing carboxylate polymers, and mixtures thereof, then alkalizing by adding a water-soluble amine compound, preferably diethylaminoethanol. This method produces a product with low weight percent Zr in the product.
The following patent and articles disclose hydrothermal production of zirconia particles as precipitates: Somiya, S. et al., "Hydrothermal Processing of Ultrafine Single-Crystal Zirconia and Zirconia Powders with Homogeneous Dopants", Advances in Ceramics, vol. 21, The American Ceramic Society, Inc., 1987; U.S. Pat. No. 4,619,817 (Stanbaugh et al.), issued Oct. 28, 1986; and Tani, E. et al., "Hydrothermal Preparation of Ultrafine Monoclinic ZrO.sub.2 Powder", Communications of the America Ceramic Society, C-181, Dec. 1981.
The following patents disclose various high solids methods for forming zirconia sols from ZrOCl.sub.2.8H.sub.2 O: U.S. Pat. No. 2,984.628 (Alexander et al.). issued May 16. 1961; and EP Patent Application No. 229,657 (Kato), issued July 22, 1987. The Alexander et al. patent discloses the formation of a 20% solids suspension (66% in the dispersed phase) by autoclaving for 4 hours at 150.degree. C. In Kato, the mixture was sealed in a PTFE container and heated in an autoclave at 200.degree. C. for 5 days.
U.S. Pat. No. 4,719,091 (Wusirika), issued Jan. 12, 1988, discusses the preparation of zirconia crystals, but this method takes between 2-5 days to complete and uses different starting materials than the present invention.
The present invention provides the following advantages over the aforementioned patents and articles:
(1) The present invention uses a different starting material, i.e., a zirconium acetate solution, which buffers the pH in a range where the use of TEFLON or expensive metallurgy is not required. Running this reaction using starting materials such as ZrOCl.sub.2.8H.sub.2 O or zirconium sulfate (involving Cl.sup.31 and sulfate ions) led to corrosion of standard metallurgy, e.g., 316 stainless. Even in a glass-lined reactor the stirring hardware corroded badly. In addition, this weaker organic acid decreases safety hazards and burns off cleanly, i.e., no NO.sub.x or SO.sub.2 production.
(2) The present invention greatly reduces processing time. Other conventional processes for producing similar particle morphologies take between 2-5 days; whereas the present invention requires only about 5 hours.
(3) The present invention can produce a monodispersed sol from high initial solids loading, i.e., zirconium acetate is present in an amount between 0.2 to 0.65 moles/liter based on zirconia.
(4) The resultant zirconia sol can be concentrated above 30 weight percent with stability beyond two months.
(5) The process of the present invention is controlled by the addition of a large molar excess of acetic acid, not just as a pH control, but also for producing a stable end product.
The present invention therefore overcomes the abovementioned deficiencies, as well as provides additional advantages which shall become apparent as described below.